Water

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Partitioning of Small Amphiphiles at Surfactant Bilayer/ Water Interfaces: An Avoided Level Crossing Muon Spin Resonance Study Robert Scheuermann,† Ian M. Tucker,‡ Herbert Dilger,§ Ed J. Staples,‡ Gary Ford,‡ Stuart B. Fraser,‡ Bettina Beck,§ and Emil Roduner*,§ Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen, Switzerland, Unilever Research and Development, Port Sunlight, Wirral CH63 3JW, U.K., and Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany Received November 21, 2003. In Final Form: January 6, 2004 The temperature-dependent variation of local environment and reorientation dynamics of the small amphiphile 2-phenylethanol in lamellar phase dispersions of the dichain cationic surfactants, 2,3diheptadecyl ester ethoxypropyl-1,1,1-trimethylammonium chloride (DHTAC) and dioctadecyldimethylammonium chloride (DODMAC), and the nonionic surfactant, tetra(ethylene glycol) n-dodecyl ether (C12E4), have been determined using avoided level crossing muon spin resonance spectroscopy (ALC-µSR). For cosurfactant radicals the hydrophobic or hydrophilic character of the surrounding media can be determined from their magnetic resonance signatures. Comparison of the three different bilayer-forming surfactant systems shows that the ALC-µSR technique is able to distinguish both major and subtle differences in the partitioning of the cosurfactant radicals between the different systems.

Introduction At high concentrations, surfactant self-aggregation leads to the formation of “liquid crystalline” structures. Surfactants with low spontaneous curvature in water form “lamellar” liquid crystalline phases where the surfactant self-assemblies adopt the form of arrays of regularly separated bilayer “sheets” separated by solvent.1-3 Subtle changes in microstructure can have dramatic effects on macroscopic behavior, manifesting in changes in rheology, and can be realized by changes in surfactant and electrolyte concentrations and/or the addition of cosurfactants. Cosurfactant action is valuable where, for example, a high curvature surfactant phase may be required at point of use, but constraints imposed by formulation, storage, and dispensing demand a microstructure which requires a very different spontaneous curvature. The mixing behavior of surfactants at interfaces has been studied extensively.4,5 Some studies have been performed on dichain cationic molecules similar to those featured in the present work.6,7 In these studies the * To whom correspondence may be addressed. E-mail: e.roduner@ ipc.uni-stuttgart.de. Phone: +49 (0)711 685 4490. Fax: +49 (0)711 685 4495. † Paul Scherrer Institut. ‡ Unilever Research and Development. § University of Stuttgart. (1) Mitchell, D. J.; et al. J. Chem. Soc., Faraday Trans. 1983, 79, 975-1000. (2) Aggregation Processes in Solution; Wyn-Jones, E., Gormally, J., Eds.; Elsevier: Amsterdam, 1983. (3) Hamley, I. W. Introduction to Soft Matter: Polymers, Colloids, Ampiphiles and Liquid Crystals; J. Wiley & Sons: London, 2000. (4) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 337. (5) Rubingh, D. N.; Holland, P. M. In Cationic Surfactants, Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series 37; Marcel Dekker: New York, 1990. (6) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Weller, A.; Penfold, J.; Webster, J. R. P.; Sivia, D. S.; Rennie, A. R. Langmuir 2001, 17, 5858. (7) Penfold, J.; Staples, E.; Tucker, I.; Soubiran, L.; Creeth, A.; Hubbard, J., Phys. Chem. Chem. Phys. 2000, 2, 5230-5234

surfactants are in the dilute state, and the addition of nonionic surfactant compensates for the very low critical micelle concentration, thus greatly reducing the time required for the adsorbed layer at the air/liquid interface to come to equilibrium. To date there have been two other studies directed toward the cosurfactant partitioning in the concentrated state.8,9 In the first case small-angle neutron scattering (SANS) allied with solvent contrast matching was used in order to determine the partitioning of cosurfactants in didodecyldimethylammonium bromide (DDAB) bilayer interfaces. Lately our preliminary work9 demonstrated the feasibility of a new approach based on avoided level crossing muon spin resonance spectroscopy to the study of this branch of soft condensed matter. In this paper we address the problem of cosurfactant partitioning at surfactant bilayer/water interfaces in more detail by a refinement of the polarity scale used to describe the partitioning and by extension of our studies to two other bilayer systems. Avoided Level Crossing Muon Spin Resonance Spectroscopy (ALC-µSR) ALC-µSR is a sensitive magnetic resonance technique for determining the reorientational dynamics and local environments of molecules which can form muonated radicals.10 Spin labeling is achieved by muonium addition to an unsaturated chemical bond or, as in the present work, to an aromatic system (see Figure 1). Muonium (Mu), the bound state of an electron and a positive muon, can be considered as a light isotope of hydrogen. The high sensitivity of the method arises from the facts that the muonated radical thus formed has a spin label with a very high polarization (almost 100%), combined with its decay characteristic leading to detection via a highly efficient single particle counting method. Any changes in (8) Ricoul, F.; Dubois, M.; Zemb, T. J. Phys II, 1997, 7 (1), 69-77. (9) Scheuermann, R.; Tucker, I. M.; Creeth, A. M.; Dilger, H.; Beck, B., Roduner, E. Phys. Chem. Chem. Phys. 2002, 4, 1510-1512. (10) Roduner, E. Chem. Soc. Rev. 1993, 22, 337-346.

10.1021/la036188s CCC: $27.50 © 2004 American Chemical Society Published on Web 02/17/2004

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Figure 2. Structure of the surfactant molecules studied in this work.

Figure 1. Formation of the three isomers of the muonated cyclohexadienyl radical derived from 2-phenylethanol by muonium addition to the phenyl ring. The methylene ∆0 resonances studied in this work involve the proton bound to the same carbon atom as the muon.

geometry, spatial requirements, polarity, and polarizability of the tracer molecules on Mu addition to the phenyl ring are small. Therefore the muon spin label is considered a nearly nonperturbing probe, certainly far less compared with the conventional case where, e.g., a nitroxide free radical is attached as a spin label. The hyperfine coupling of a three-spin-1/2 system (muon, proton, electron) in the muonated radicals described herein governs the time evolution of the muon spin polarization at avoided crossings of magnetic energy levels in high magnetic fields. This time evolution (an oscillation between eigenstates) is observed as a sharp peak in the field dependence of the time-integrated muon spin polarization. The position of these resonances is determined by the muon and the proton hyperfine coupling constants which reflect the electron spin densities at these nuclei. They are furthermore affected by the polarity of the surroundings, via polarization of the unpaired electron spin density. Two types of resonances are discussed. One of them, here called ∆1 (based on a magnetic resonance selection rule, as detailed in ref 11), is observed only in anisotropic environments. It is permitted by the muon-electron magnetic dipolar interaction tensor, and its width and intensity reflect the extent of motional averaging of this tensor. In the present context the mere presence of the ∆1 resonance is taken as a sensitive indicator of small anisotropy of the radical reorientational motion on a time scale of typically 50 ns. In this paper we build on previous work on DHTAC9 and demonstrate the sensitivity of the ALC-µSR method in two other bilayer-forming surfactant systems, one where the surfactant mesophase has a higher charge density (DODMAC) and the other where the headgroup is virtually uncharged (C12E4). 2-Phenylethanol (PEA) was chosen as the cosurfactant for which the partitioning at the sur(11) Roduner, E.; Stolma´r, M.; Dilger, H.; Reid, I. D. J. Phys. Chem. A 1998, 102, 7591-7597.

factant bilayer/water interface is studied. With respect to magnetic properties and reorientation dynamics (deduced from changes in the line shapes) the muonated radical derived from this small amphiphile molecule is expected to behave very similar as the well-studied cyclohexadienyl radical C6H6Mu.11 Experimental and Data Analysis Sample Preparation. Ultrapure water was obtained from a MilliQ/Milli-RO system and stored in glass-stoppered glass vessels to avoid pickup of impurities prior to use. Surfactants were obtained in recrystallized form from Unilever Research Port Sunlight, their structual formulas are shown in Figure 2. All other chemicals were analytical grade (high purity) obtained from Sigma-Aldrich. The 15 wt % dispersions of the cationic surfactants dioctadecyldimethylammonium chloride (DODMAC) and 2,3-diheptadecyl ester ethoxypropyl-1,1,1-trimethylammonium chloride (DHTAC) were prepared in deoxygenated water by melting the surfactant and shear mixing the melt through a narrow orifice. Water, surfactants, and any other components were first deoxygenated by bubbling dry nitrogen gas through the liquid (surfactants in their molten state) for approximately 30 min. A 35% w/w dispersion of the nonionic surfactant tetra(ethylene glycol) n-dodecyl ether (C12E4) was deoxygenated in a similar manner. The solution was first warmed to 45 °C and allowed to cool naturally to ensure complete dispersion. The cooled solutions were stored under oxygen-free conditions. Due to its high vapor pressure, PEA was prepared oxygen free by repeatedly freezing the solution and evacuating the vapor until degassing ceased. Under oxygen-free conditions (i.e., in a glovebox) 40 mM PEA was added at room temperature and cold-mixed by shearing through a narrow orifice. PEA solutions (40 mM) were also prepared in 0.17 M tetramethylammonium chloride solution (concentration equivalent to the 15% DHTAC dispersion) and in bulk ethyl acetate. The resultant dispersions were injected into brass cells of 50 mm inner diameter and 17 mL internal volume and sealed with a 50 µm brass window. ALC-µSR. ALC-µSR experiments were performed at the spectrometer at beamline πE3 of the Swiss Muon Source, “SµS”, Paul Scherrer Institut, Villigen, Switzerland. Resonance spectra (i.e., the field dependence of the time-integrated muon spin polarization) were recorded over the field ranges appropriate for both types of resonances, typically 17500-21500 G. Measurements were made over the temperature range 25-75 °C for DHTAC and DODMAC and from 8 to 30 °C for C12E4 dispersions. Raw data were corrected for the field-dependent background by subtracting the spectrum from an isodense liquid producing no resonance features in the field ranges of interest (pure water in this instance). Since this correction depends sensitively on a number of instrumental parameters, we refrain in the present case from an interpretation of a remaining apparent field-

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Figure 3. Fourier power spectrum of bulk PEA at 35 °C in a transverse magnetic field of 3000 G. The temperature dependence of the muon coupling constant for the three isomers is linear; the parameters are given in Table 1. Table 1. Linear Temperature Dependence of the Muon Hyperfine Coupling Constants of the Three PEA-Mu Isomers Aµ (35 °C) (MHz) dAµ/dT (MHz/K)

ortho

para

meta

494.5 -0.063

500.8 -0.085

511.4 -0.070

dependent baseline. Least-squares fitting of multiple Lorentzians was applied to the corrected data using a procedure based on the MINUIT function minimization library.12 Determination of the Muon Hyperfine Coupling Constants. An established procedure in muon spin resonance,13 transverse field experiments permit the identification of muonated isomers with different coupling constants. Oxygen-free samples of cosurfactants were prepared in special round glass bulbs of 30 mm diameter and sealed against vacuum with a 30 mm long neck. Using the GPD spectrometer at PSI (area µE1), a transverse field (i.e., perpendicular to the initial muon spin polarization) of typically 3000 G was applied. The muon coupling constant of each muonated cosurfactant isomer is determined from the analysis of the precession signals, each species giving rise to one pair of frequencies split by the muon coupling constant. The hyperfine coupling constant thus determined facilitates prediction of the ∆1 resonance field according to the equation14

B(∆1) )

|

|

Aµ Aµ 2γµ 2γe

(1)

where Aµ denotes the muon coupling constant and γµ and γe denote the gyromagnetic ratios of the muon and the electron, respectively. A typical value for the methylene proton coupling Ap in cyclohexadienyl-type radicals is 125 MHz, 14 and the corresponding ∆0 resonance field can be calculated14 from

B(∆0) )

|

Aµ - Ap

2(γµ - γp)

-

|

Aµ + Ap 2γe

(2)

Figure 3 shows the Fourier power spectrum of bulk PEA at 35 °C obtained in a transverse magnetic field of 3000 G. The muon coupling constant depends linearly on temperature; the parameters are given in Table 1. However, this method works only at sufficiently high concentrations of tracer molecules larger than about 100 mM.14

Results and Discussion A Polarity Scale Reflecting the Local Environment of the Mu Adduct Radical of PEA. Three different (12) MINUIT-Function Minimization and Error Analysis; CERN Program Library Entry D506 (CERN, Geneva, Switzerland). (13) Roduner, E.; Fischer, H. Chem. Phys. 1981, 54, 261-276. (14) Roduner, E. The Positive Muon as a Probe in Free Radical Chemistry. Potential and Limitations of the µSR Techniques; Lecture Notes in Chemistry 49; Springer: Heidelberg, 1988.

Figure 4. Methylene ∆0 resonances of PEA-Mu in liquid water (bottom) and molten octadecane (top) at 35 °C. The relative resonance positions of the three individual isomers (ortho, para, meta) allow to define a polarity scale calibrated on these two solvents (0% and 100% “aqueous character”).

isomers of the muonated cyclohexadienyl radical derived from 2-phenylethanol are formed by Mu addition to the phenyl ring in the ortho, para, and meta positions with respect to the ethyl substituent (Figure 1). Each of the three isomers gives rise to a methylene ∆0 resonance (involving the proton bound to the same carbon atom as the muon).11 The assignment of the individual resonance lines is made on the basis of substituent effects on the hyperfine coupling constants in monosubstituted radicals14 and the relative amplitudes. The resonance spectra obtained with a 40 mM solution of PEA in water and molten octadecane at 35 °C are shown in Figure 4. The corresponding resonance positions of the individual isomers are clearly shifted against each other due to the different polarization of the unpaired electron spin density distribution by the two solvents. It is assumed that this polarization is mostly due to hydrogen bonds to the delocalized π-system of the cyclohexadienyl ring. On this basis, a separate polarity scale can be defined for each isomer based on the “aqueous character” of the environment by calibration on water (100% aqueous) and C18H38 (0% aqueous). This approach is supported by further experiments on different alcohols with increasing alkyl chain lengths (i.e., decreasing aqueous character according to our definition) and methanol-water mixtures. A straightforward approach to describe the polarity of a solvent or solvent mixture is the concentration of OH groups. In this particular case, the term “polarity” is used as equivalent to “ability of hydrogen bond formation”. Only one OH group is counted for each water molecule, assuming that it can form only one hydrogen bond to the same PEA molecule. It is believed that the unpaired electron spin density is affected most by this effect. As shown in Figure 5a the polarity sensed by the ortho and meta isomers depends in good approximation linearly on the OH concentration. For the methanol-water mixtures

Small Amphiphiles at Surfactant Interfaces

Figure 5. Polarity diagram for 40 mM PEA in various solvents, obtained from the relative resonance positions of PEA-Mu compared to H2O and C18H38 (Figure 4): 0, o-PEA-Mu; O, m-PEA-Mu; 4, p-PEA-Mu. (a) Dependence on concentration of OH groups. (b) Dependence on the dielectric constant .

this means that water is distributed homogeneously and does not accumulate near the probe molecule. Bulk ethyl acetate has line positions which correspond to between 30 and 40% aqueous character but has no OH group to form hydrogen bonds to the radical and is off the line of Figure 5. This demonstrates that besides hydrogen bonding there is another mechanism, probably dielectric polarization. In Figure 5b the aqueous character derived from the ∆0 resonances is plotted against the dielectric constant  (values taken from refs 15 and 16). In good approximation the dependence on  is also linear. In plain alcohols the para isomer seems to sense a more hydrophobic environment than the other two isomers, whereas it is perfectly on line as soon as some water is present. The deviation of the para isomer is presently not understood. It may arise from a competition of intramolecular hydrogen bonds of PEA-Mu (which may form to a different extent for the three isomers) with intermolecular hydrogen bonds to the solvent. The influence of solvent polarity and hydrogen bonding has been well investigated for nitroxide radicals.17 It was found that the isotropic hyperfine coupling constant of the nitrogen atom increases monotonically but not linearly with the solvent dielectric constant, and that hydrogen bonds have a significant additional effect. Since the constitution of the nitroxide radicals is quite different from that of cyclohexadienyl radicals it is interesting to see (15) Wohlfarth, Ch. In Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2002; Section 6, pp 6-153. (16) Dielectric constant measured with a BI-870 dielectric constant meter (http://www.bic.com/BI-870.htm). (17) Owenius, R.; Engstro¨m, M.; Lindgren, M.; Huber, M. J. Phys. Chem. A 2001, 105, 10967-10977.

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Figure 6. ALC resonance spectra of PEA in DHTAC at various temperatures. In Lβ only ∆0 resonances are detected; the appearance of ∆1 resonances along with a discontinuity of the temperature dependence of the ∆0 resonances (dashed lines indicate the T-dependence of the different isomers due to thermal excitations of molecular vibrations) indicates a change of the polarity and the local environment above the phase transition.

that qualitatively a very similar behavior is found for the two species. The Mu Adduct Radical to PEA in DHTAC. This system forms a simple (i.e., non-interdigitated) lamellar phase structure as determined by small-angle X-ray scattering (SAXS). At this concentration the d-spacing is 302 ( 4 Å from which the bilayer thickness (headgroups plus chains) of 52 Å is determined. SAXS and differential scanning calorimetry indicate that this system undergoes a phase transition from LR to Lβ at about 55-57 °C. The partitioning of the muonated cyclohexadienyl radical derived from 2-phenylethanol (PEA-Mu) in a lamellar phase dispersion of DHTAC has been reported previously.9 An abridged version of the results is shown here to facilitate reader ease (Figure 6). In the low temperature (Lβ) phase, where the alkyl chains in the bilayer are “frozen”, the absence of ∆1 resonances and the positions of the ∆0 resonances close to the value observed for PEA-Mu in water indicate that the tracer resides in a mostly aqueous environment between the bilayer sheets, where it undergoes isotropic reorientation. The resonance field values, Bres(T), of the three isomers decrease linearly with increasing temperature, reflecting the temperature dependence of the unpaired electron spin density at the muon due to the excitation of molecular vibrations. The discontinuous jump of Bres(T) to lower magnetic fields at the phase transition is attributed to a change of the local environment of the tracer to a less polar one: in the high-temperature phase (LR) the tracer can penetrate the surfactant bilayer, where the alkyl chains are “molten”. The value of about 55% for the aqueous character of the environment of ortho and meta PEA-Mu (exact values are given in Table 2) can be interpreted in two ways. First

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Table 2. Polarity (in % aqueous character) Sensed by the PEA-Mu Isomers in the Systems Studied in This Work DHTAC DODMAC PEA in ethyl acetate N(CH3)4Cl ortho

32

96

LR

Lβ

63

LR 62

93 para

12

99

15

meta

37

99

53

Lβ

51 62

4 96

56 10

4 51

92

C12E4 LR L1

17 41

51

46

it is possible that the radical exchanges rapidly between the water phase and the bilayer, resulting in an average value for the polarity of its environment. Second the radical resides within the bilayer close to the headgroup which contains some water. It was found previously that there is a gradient of water that penetrates along the lipid chain deeply into phospholipid membranes.18 To discuss the first possibility, we estimate the one-dimensional root-meansquare (rms) displacement by diffusion, 〈x2〉1/2 ) (2Dτµ)1/2. Using D ) 1 × 10-9 m2 s-1 as a typical value19 for a diffusion coefficient of a small molecule in water at 25 °C and the muon lifetime, τµ ) 2.2 µs, gives a rms displacement of 66 nm in water. Even if the local viscosity is increased by 100-fold in the bilayer, the consequential reduction in diffusion coefficient is not sufficient to prevent the small molecule from random walking over a distance which is still larger than the bilayer diameter. The local viscosity would have to be around 5000 to 10 000 times that of water before this effect should become significant. It is therefore reasonable to assume rapid exchange of the tracer molecule between the two environments over the muon lifetime. This estimate is compatible with experimental determinations of exchange rates for dialkyl nitroxides in micelles.20 For the case when water is present in the bilayer, the mean residence time of the radical in the bilayer has to be longer in order to yield the same value for the average polarity. The low value obtained for para-PEA-Mu indicating a more hydrophobic environment must not be interpreted as a result of deeper incorporation of this isomer into the alkyl chains as this deviation from the behavior of the two other isomers is also observed in isotropic phases (see Figure 5). The muon coupling constants derived from the ∆1 resonance fields according to eq 1 in the LR phase are slightly larger than the values obtained for bulk PEA, as expected from the positions in the polarity diagram constructed from the ∆0 resonance positions.9 From this we conclude that a more polar environment according to our definition leads to a larger muon and proton coupling constant. The muon coupling of PEA-Mu in the Lβ phase cannot be determined, due to the absence of ∆1 resonances, and the low concentration of tracer molecule does not allow to determine Aµ for this system in transverse field measurements.14 In the present work more detailed information on the preferred location of the tracer molecule was obtained from ALC-µSR experiments on solutions of 40 mM PEA in both tetramethylammonium chloride (concentration equivalent to 15% DHTAC dispersion) and ethyl acetate (bulk). These molecules were used to model the polar headgroup and the ester bridge between the headgroup and the alkyl chains in DHTAC, respectively. The resulting (18) Earle, K. A.; Moscicki, J. K.; Ge, M.; Budil, D. E.; Freed, J. H. Biophys. J. 1994, 66, 1213-1221. (19) Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1995; Section 6, pp 6-257. (20) Brigati, G.; Franchi, P.; Lucarini, M.; Pedulli, G. F.; Valgimigli, L. Res. Chem. Intermed. 2002, 28, 131-141.

Figure 7. Polarity diagram for PEA-Mu in DHTAC, DODMAC, and related isotropic systems (ethyl acetate and 0.17 M aqueous tetramethylammonium chloride solution) used to model parts of the DHTAC molecule: O, octadecane; b, water; 9, DHTAC; 4, DODMAC; 0, tetramethylammonium chloride; 1, ethyl acetate. The polarity values are obtained from the relative resonance positions corrected for their intrinsic temperature dependence.

polarity diagram (corrected for the linear temperature dependence of the resonance fields in both phases) is shown in Figure 7. The resonance positions of PEA-Mu in the tetramethylammonium chloride solution are close to the values obtained in H2O and also in the Lβ phase of DHTAC. This demonstrates that the tetramethylammonium chloride part of the headgroup cannot be distinguished from the aqueous environment, based on the shift of the resonance field. In contrast, PEA-Mu in the ethyl acetate solution shows a similar or even less aqueous environment than for DHTAC in the LR phase. This indicates that PEAMu resides near the headgroup rather than penetrating deeply into the bilayer and that this region may also contain some water. This suggestion of water content in the headgroup region is consistent with the models regularly used to determine the water distribution function when analyzing surfactant adsorbed layers by neutron reflectivity.21 Our interpretation of the preferred location of PEA-Mu in DHTAC is further supported by the results of paramagnetic quenching experiments for which a solution containing paramagnetic Ni2+ ions is added to the lamellar phase dispersion (Figure 8). Spin exchange leads to broadening of the resonance lines. At the Ni2+ concentrations used in the present experiment (0.063 M) at 35 °C the ∆0 resonances of PEA-Mu in Lβ-DHTAC disappear completely. In the LR phase the ∆0 resonances persist but, in common with the ∆1 resonances, these are extremely broadened. This shows that the nickel ions still have access to PEA-Mu in the LR phase, but to a significantly lower extent than in the Lβ phase. Either, as suggested by the results obtained on PEA-Mu in N(CH3)4Cl shown above, in the LR phase the tracer resides close to the headgroup (21) Lu, J.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143-304.

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Figure 8. Paramagnetic quenching of the ALC resonances by Ni2+ addition in the Lβ phase (35 °C, middle). Addition of diamagnetic Ca2+ instead of Ni2+ does not affect the amplitudes (35 °C, bottom). In the LR phase (75 °C, top) both types of resonances are present, although with large widths due to electron spin exchange. This implies that the effective nickel concentration at the radical is less in the LR than in the Lβ phase.

region, or it rapidly exchanges between the bilayer and the water layer. In control experiments where diamagnetic Ca2+ ions of the same concentration were added to the Lβ phase of a DHTAC dispersion such a line broadening was not observed (see Figure 8). A small reduction (15 G) in the resonance positions was the only consequence, which we ascribe to reduction in cosurfactant solubility due to the addition of electrolyte. As a further effect, as a consequence of restricted molecular motion of the radical, ∆1 resonances appear above the LR to Lβ phase transition at about 50 °C. These resonances are very narrow compared to their full static width of about 520 G, indicating very fast but not isotropic averaging of the dipolar interaction.22 The amplitudes of these signals permit a further quantification of the extent of averaging. For a fully static case, the ∆1 resonances have a considerable higher amplitude than those of selection rule ∆0.11 From comparison with Monte Carlo simulations using the code of ref 23 the experimentally observed amplitudes at temperatures above 50 °C (Figure 4) are estimated to correspond to an effective muon hyperfine anisotropy of about 0.2 MHz, averaging out more than 97% of the full static anisotropy of typically 6.8 MHz for the Mu-substituted cyclohexadienyl radical and thus reducing the amplitude to about 40% (see also Figure 3 in ref 22). Part of this averaging probably occurs in the aqueous phase. However, the experimentally observed line widths are about two to three times larger than expected from the simulations. Therefore, an additional dynamic process, e.g., chemical reaction of the radical or electron spin exchange from residual oxygen, has to be taken into account in order to explain the experimental data quantitatively. It is significant in this context also that the para isomer loses less polarization than the ortho, and in particular than the meta isomers: inside the bilayer there is preferential rotation about the long axis of the molecule. Considering the orientation of the dipolar part of the hyperfine tensor,22 this motion retains more of the anisotropy for the para than for the other isomers. PEA-Mu in DODMAC. DODMAC consists of molecules with a smaller headgroup and a higher charge density than DHTAC and forms an interdigitated bilayer struc(22) Roduner, E. Hyperfine Interact. 1990, 65, 857-872 1990. (23) Tregenna-Piggott, P. L. W.; Roduner, E.; Santos, S. Chem. Phys. 1996, 203, 317-337.

Figure 9. ALC spectra of PEA in DODMAC at different temperatures. The striking difference compared to DHTAC is the presence of ∆1 resonances in both phases. At the lowest temperature (25 °C) the splitting of the meta-∆1 resonance indicates a second phase transition.

ture.24,25 The ALC-µSR results obtained on PEA-Mu in a DODMAC phase dispersion are displayed in Figure 9; the temperature corrected polarity values are shown in Figure 7. The most obvious difference between the two systems is the existence of ∆1 resonances in both the LR and Lβ phase of DODMAC. From this we conclude that in DODMAC the PEA-Mu tracer is axially aligned within the bilayer in both the LR and Lβ phase. The relative intensities of the ∆1 resonance lines are much larger in DODMAC than observed in the simple lamellar structure of DHTAC, revealing a significantly higher order (larger residual anisotropy) of PEA-Mu. This example again demonstrates the sensitivity of the ALC-µSR method to different mesophase structures. The polarity sensed by the tracer in the Lβ phase of DODMAC within errors is identical to that in the LR phase of DHTAC. This is interesting in view of the fact that DODMAC, in contrast to DHTAC, does not contain an ester group. It is unlikely that the effect of the absence of this group is compensated by a higher fraction of water molecules within the bilayer. It therefore means that in a dynamic equilibrium the tracer molecule spends a somewhat larger fraction of time in the aqueous phase than in the DODMAC bilayer, relative to the DHTAC system. The isotropic muon coupling constant of m-PEA-Mu determined from the ∆1 resonances is shown in Figure 10. There is evidence for a small discontinuity at the phase transition. The slightly larger value of Aµ in the LR phase (24) Tucker, I. M. Unpublished data, 15% dispersion of DODMAC has a d spacing of 185 ( 5 Å, which implies a hydrocarbon layer thickness of 30 Å suggesting interdigitation. (25) Walsh, J. Ph.D. Thesis, University of Salford, Manchester, U.K., in preparation.

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Figure 10. Isotropic muon hyperfine coupling for m-PEA-Mu in DODMAC, deduced from ∆1 rsonance positions (eq 1). The LR/Lβ phase transition is observed as a discontinuity of the temperature dependence of Aµ (dotted lines).

Figure 11. ALC resonance spectra of PEA in C12E4 at 8 °C (micellar phase L1, bottom) and 30 °C (lamellar phase LR, top). The very broad feature in the ∆1 region of the L1 phase is indicative of averaging of the hyperfine anisotropy due to micellar motion (the radical is already incorporated within the micelle).

than expected from a linear extrapolation of the lowtemperature values indicates a more polar environment sensed in LR. This can be explained by additional water penetration into the bilayer18 in the high-temperature phase. At the lowest temperature measured (25 °C) a splitting of the meta-∆0 resonance line is observed. This is a further strong indication for the existence of a second Lβ phase forming in DODMAC at this temperature.25 PEA-Mu in C12E4. At 35% w/w in water C12E4 undergoes a phase transition from the micellar phase (L1) to the lamellar phase LR at about 15-20 °C.1 At 8 °C (micellar phase) only the three ∆0 resonances of the three PEA-Mu isomers are clearly observed (Figure 11). Their position reveals a polarity of ca. 50% aqueous character, which means that the tracer molecule spends a significant fraction of time within the surfactant phase. However, in the region of expected ∆1 resonances a very broad feature is hardly discriminated from the background. This could be due to rotational motion of the micelle. From comparison with Monte Carlo simulations23 a correlation time of ca. 5 ns (i.e., an order of magnitude smaller than the inverse static hyperfine anisotropy of typically 50 ns) is estimated under the assumption that the averaging occurs solely by rotational diffusion. Assuming further a value of 1 mPa‚s for the viscosity of the dispersion, the micelle radius is calculated to be 1.7 nm, in good agreement with the length of a C12 alkyl chain (1.6 nm) forming the core of a micelle. This, of course, is only an order of magnitude estimate

Scheuermann et al.

regarding the uncertainties of the values for the hyperfine anisotropy, the correlation time, and the viscosity. Increasing the temperature to 30 °C leads to the appearance of sharp ∆1 resonances and, in addition, shifts the ∆0 resonance positions to lower fields than would be anticipated from the temperature dependence alone implying that the PEA-Mu radical in the average resides in a slightly more hydrophobic local environment in the LR phase. The ∆1 resonances are very narrow and have low amplitude, typical of small anisotropy, indicating a weak association between the tracer molecule and the bilayer, or rapid (but not isotropic) reorientation of the radical within the bilayer. These results show similar trends as in the DHTAC system; however it is clear that in the micellar state the PEA resides in an environment which is very different from bulk water. Consistent with our earlier observations the para isomer seems to experience a much more hydrophobic environment than the other isomers. Given that we have similar results in two very different surfactant systems and also in surfactant-free isotropic solvents, this observation must be generic and has little to do with hydrophobicity of the environment. The ortho and meta isomers of PEA-Mu experience what approximates to 50% aqueous character, and there is a decrease of around 6% for all isomers on formation of the C12E4 LR phase, which is just outside of the error. We therefore conclude that in C12E4 the PEA resides within the micelle and is more fully incorporated into the lamellar phase. In the latter state the local environment is more hydrophobic than that for the cationic charged bilayers. This may be somewhat unexpected, since the glycol ether environment is able to stabilize OH groups by hydrogen bonding and may therefore have been expected to be slightly more favorable for both PEA and guest water molecules. Conclusion The temperature-dependent variation of local environment and reorientational dynamics of the small amphiphilic molecule 2-phenylethanol in lamellar phase dispersions of dichain cationic surfactants (DHTAC and DODMAC) and in C12E4 have been determined using avoided level crossing muon spin resonance spectroscopy. The variation in position of ∆0 magnetic resonance signatures is related to hydrophobic/hydrophilic character of the surrounding media. Step changes in these fundamental resonance positions are observed at the onset of the LR/Lβ phase transition in the simple surfactant bilayer formed in DHTAC, consistent with the more liquid-oillike character of the LR lamellar phase. Using spin exchange to suppress the resonance lines, we have shown either that the tracer molecule resides close to the headgroup region and has access to some water or that there is a rapid dynamic exchange between aqueous and nonaqueous environments. Following calibration in 100% aqueous to 100% nonaqueous (i.e., plain octadecane) environments and compensation for the known temperature dependence, quantification of the affinity for a particular environment type becomes possible. By means of extension to other systems, we reinforce our preliminary interpretations.9 We show that for PEA, where the polarity of the tracer molecules can be assessed in terms of aqueous/nonaqueous character, we can not only determine the local environment in different mesophases but also demonstrate that the method is sensitive to subtle differences in these local environments. We show that the LR phase approximates to between 40 and 60% aqueous character, indicating that

Small Amphiphiles at Surfactant Interfaces

the tracer is coadsorbed at the oil-water interface. Furthermore, by considering the character of the headgroup region, we find that the tracer resides in a more aqueous environment, i.e., closer to the water than to the oil. These findings for the local microenvironment of the radical also hold for the DODMAC system which forms an interdigitated bilayer. In contrast to DHTAC the tracer radical is not expelled from the bilayer in the temperature range studied, a conclusion drawn from the all present ∆1 resonances, clearly demonstrating the presence of an anisotropic environment for the radical in both the LR and the Lβ phases. Further confirmation of our interpretation is shown by repeating these measurements using the nonionic surfactant C12E4 as the structurant. On transition from L1 (micellar) to LR (lamellar phase), we again observe a shift in the resonance position of the ∆0 lines, and the appearance of a very sharp triplet at lower field again

Langmuir, Vol. 20, No. 7, 2004 2659

indicates axial confinement of the tracer molecule. Again from the ∆0 shift we conclude that the cosurfactant is located within the micelle. Further applications of the ALC-µSR method could involve the distribution of drug molecules between the aqueous environment of the cell fluid and the cell membrane or the lamellar double layers of liposome drug carriers.26 Acknowledgment. This work is based on experiments performed at the Swiss Muon Source (SµS), Paul Scherrer Institut, Villigen, Switzerland. We cordially thank our hosts at the facility at the Swiss Muon Source and take pleasure in acknowledging the contributions of Drs. A. Raselli and U. Zimmermann. LA036188S (26) Roduner, E. Physica B 2003, 326, 19-24.